The invention pertains to a crucible for the inductive melting or super-heating of metals, alloys, or other electrically conductive materials with palisades, i.e., wall segments, of approximately equal length, which are arranged vertically, parallel to, and a certain distance away from each other around a circle so as to surround the melt; with a plate-shaped or ring-shaped part to hold the palisades, provided at the bottom ends of the palisades, at least parts of the palisades being provided with cavities or channels, through which a coolant flows; and with an induction coil through which an alternating current flows, and which surrounds the palisades a certain distance away from their outside surfaces.
A method for melting difficult-to-melt metals, especially tantalum, tungsten, and thorium as well as alloys of these metals in a water-cooled container (DE 518,499) is known, where the container consists of material, e.g., quartz glass, copper, or silver, with a melting point lower than that of the material to be melted, and where the energies required for melting and for cooling the container are supplied in such a way that the material is completely melted without being contaminated in any way by the material of the crucible. The crucible itself can be heated by means of an induction coil, and a current which rotates around the crucible is prevented by constructing the crucible out of individual segments, which are separated from each other by an insulating layer made of a material such as mica.
A high-frequency induction crucible is also known, which is made up of a plurality of palisades, all of which are arranged vertically on a circular base plate and which together form a hollow cylinder (U.S. Pat. No. 3,461,215). Cooling water flows through all of the current-conducting and heat-conducting palisades, which are surrounded by an induction coil. The base plate of ceramic material is provided with a sealed opening, through which the melt can be removed. Strips of insulating material are inserted between the palisades.
In addition, a crucible for the slagless melting of highly pure, reactive metals in a vacuum chamber is also known (EP 0 276 544), in which cooling water flows through the palisades, which are surrounded by the induction coil, and which are rigidly screwed to a circular, disk-shaped base plate. The tubular palisades are separated from each other by slots, whereas at the same time they are all connected electrically to each other by way of the base plate, which is made of metal.
The known crucibles suffer from the disadvantage of comparatively poor thermal efficiency. The attempt has therefore been made to add insulating material to the melt to reduce the loss of heat through the cooled palisades, but this leads in turn to contamination, although slight, of the melt (Schippereit, et al.). This contamination of the melt material, however, is undesirable in many modern applications, so that this method has been accepted to only a limited extent by industry.
The arrangement and design of the palisades themselves as well as their electrical connections to, or their insulation from, each other have also already been the object of numerous studies. U.S. Pat. No. 3,223,519, for example, describes crucible segments which are insulated from each other, whereas in the case of the crucible according to EP 0 276 544, the palisades are set up a certain distance away from each other but are connected to each other electrically.
A slotted crucible, finally, is also known (EP-A-0,169,765), in which coolant flows through the individual palisades, which have a rectangular cross section. The individual palisades are uniformly distributed around a circle. Slots are provided between the palisades, and the interior space of the crucible consists of a regular, straight, polyhedral prism.
It is characteristic of the slotted crucible (cold-wall crucible) that, because of the high melting power/power density required and the associated forces, the bath forms a pronounced meniscus. Because of the effective principle on which the cold-wall crucible is based, it is necessary to slot the more-or-less cylindrical crucible. This slotting, however, causes severe disturbances in the magnetic field at the location of the slots, these disturbances leading to contractions or even to instabilities in the melt column. All the known designs must therefore be constructed expensively out of a large number of palisades with the goal of smoothing out the disturbances in the areas of the slots as much as possible.
The present invention is based on the task of creating a crucible of the type in question which does not need any insulating slag material and in which the amount of energy supplied to the melt can be adjusted to suit the material to be melted. In addition, the production costs are also to be reduced, and the formation of arcs prevented. Finally, this crucible is intended to make possible the formation of an especially stable melt column while excluding disturbances in the magnetic field.